Method for producing cerium oxide, cerium oxide abrasive, method for polishing substrate using the same and method for manufacturing semiconductor device

ABSTRACT

The present invention provides a method for producing cerium oxide comprising rapid heating of cerium salts to a calcining temperature to calcine them, a cerium oxide abrasive containing the cerium oxide produced by the method and pure water, an abrasive containing a slurry in which cerium oxide particles having an intensity ratio of an area of a primary peak appearing at 27 to 30° to that of a secondary peak appearing at 32 to 35° (primary peak/secondary peak) in a powder X-ray diffraction chart of 3.20 or more are dispersed in a medium, an abrasive containing a slurry in which cerium oxide particles whose bulk density is 6.5 g/cm 3  or less are dispersed into a medium, an abrasive containing a slurry in which abrasive grains having pores are dispersed into a medium, a method for polishing a substrate comprising polishing a predetermined substrate using the abrasive; and a method for manufacturing a semiconductor comprising the step of polishing by the abrasive.

TECHNICAL FIELD

The present invention relates to a method for producing cerium oxide, acerium oxide abrasive, a method for polishing a substrate using thesame, and a method for manufacturing a semiconductor device, and morespecifically relates to a method for producing cerium oxide particleswith high productivity and yield, a cerium oxide abrasive that canprovide high speed polishing without causing scratches irrespective ofthe film properties, a method for polishing a substrate using the same,and a method for manufacturing a semiconductor device having highreliability with high productivity and yield.

BACKGROUND ART

In a manufacturing process of a semiconductor device, as achemical-mechanical polishing method for smoothing an inorganicinsulating layer such as an SiO₂ insulating layer, which is formed by aplasma CVD method, or low-pressure CVD method, a CMP method has beenconventionally used. As an abrasive used for the CMP method, a colloidalsilica series abrasive or slurry using silica particles, cerium oxideparticles or the like as the abrasive grain is used.

The colloidal silica series abrasive is produced by grain-growing silicaparticles by a process of pyrolyzing silicic acid tetrachloride or thelike and pH adjusting the silica particles with an alkaline solutioncontaining no alkali metal such as ammonia, etc. However, such anabrasive has a technical problem that the polishing speed on aninorganic insulating film is not sufficient and higher polishing speedis required for practical use.

On the other hand, as a glass-surface abrasive for a photomask, a ceriumoxide abrasive has been used. Cerium oxide particles have lower hardnessthan silica particles or alumina particles, therefore it tends to causefew scratches on the surface to be polished and thereby it is useful infinishing mirror polishing. Further, as the cerium,oxide has been knownas a strong oxidant, it has chemically active properties. Thus, byutilizing these advantages of the cerium oxide, the application of it toa chemical-mechanical abrasive for the insulating film is useful.

However, when a cerium oxide abrasive is applied to inorganic insulatingfilm polishing as it is to polish a glass surface for a photomask, aproblem arises that it causes scratches to a degree that can be visuallynoticed on the insulating film surface as the diameter of the primarygrain is large. Further, there is another problem that some types of acerium oxide abrasive greatly change its polishing speed depending onthe film properties of a surface to be polished.

An object of the present invention is to provide a method of easilyproducing cerium oxide that can polish a surface to be polished such asan SiO₂ insulating film, etc., at high speed and good yield withoutcausing scratches.

Another object of the present invention is to provide a cerium oxideabrasive having cerium oxide as an essential component that can polish asurface to be polished such as an SiO₂ insulating film, etc., at highspeed irrespective of the film properties without causing scratches.

Yet another object of the present invention is to provide a method forpolishing a substrate that can polish a surface to be polished such asan insulating film, etc., at high speed irrespective of film propertieswithout causing scratches.

Still further object of the present invention is to provide a method formanufacturing a semiconductor device in which a semiconductor devicehaving excellent reliability can be manufactured with high productivityand yield.

DISCLOSURE OF THE INVENTION

The present invention relates to a method for producing cerium oxidecomprising rapid heating of cerium salts to a calcining temperature tocalcine them.

Further, the present invention relates to the method for producingcerium oxide in which the temperature rise rate of raising to thecalcining temperature is set to 20 to 200° C./min.

Further, the present invention relates to the method for producingcerium oxide in which the calcining is performed by a rotary kiln.

Further, the present invention relates to the method for producingcerium oxide in which the calcining temperature is set to 600 to 1,000°C. and the calcining time is set to 30 minutes to 2 hours.

Further, the present invention relates to a cerium oxide abrasivecontaining the cerium oxide produced by the above-mentioned method forproducing cerium oxide and pure water.

The present invention also relates to an abrasive containing a slurry inwhich cerium oxide particles having an intensity ratio of the area of aprimary peak appearing at 27 to 30° to that of a secondary peakappearing at 32 to 35° (primary peak/secondary peak) of 3.20 or more ina powder X-ray diffraction chart are dispersed into a medium.

Further, the present invention relates to an abrasive containing slurryin which abrasive grains having pores are dispersed into a medium.

Further, the present invention relates to an abrasive containing slurryin which cerium oxide particles having a bulk density of 6.5 g/cm³ orless are dispersed into a medium.

Further, the present invention relates to a method for polishing asubstrate comprising polishing a predetermined substrate using theabove-mentioned abrasive.

Further, the present invention relates to a method for manufacturing asemiconductor device comprising the step of polishing a semiconductorchip on which a silica film is formed with the above-mentioned abrasive.

BEST MODE FOR CARRYING OUT THE INVENTION

The cerium salts to be used in the present invention may include ceriumcarbonate, cerium sulfate, cerium oxalate and the like. These ceriumsalts may be hydrates. From the viewpoint of producing a cerium oxideeasily with good yield, which is an essential component of a ceriumoxide abrasive that can polish a surface to be polished such as an SiO₂insulating film, etc., at high speed without causing scratches, ceriumcarbonate is preferably used and cerium carbonate hydrate is morepreferably used as the cerium salts.

It is preferred that these cerium salts be in the form of powder interms of workability.

In the method for producing cerium oxide of the present invention, it isnecessary to rapidly heat cerium salts to raise the temperature to acalcining temperature and calcine them.

When gradual heating of the cerium salts is conducted to raise thetemperature to a calcining temperature and they are calcined, theobtained cerium oxide does not have desired performance, and a ceriumoxide abrasive using such cerium oxide is likely to cause scratches onthe surface to be polished and high speed polishing becomes difficult.

Here, the temperature rise rate of raising the temperature of ceriumsalts to a calcining temperature is preferably set to 20 to 200° C./min,and more preferably to 40 to 200° C./min.

Further, while the calcining process may be performed in a batchfurnace, it may be preferably performed by a rotary kiln.

Further, the calcining temperature is preferably set to 600 to 1,000° C.

Further, the calcining time is preferably set to 30 minutes to 2 hours.

The rotary kiln is an already known furnace, and a type of the rotarykiln is not limited. There may be mentioned, for example, a material inwhich a refractory lined cylindrical kiln is provided such that the axisof the kiln is inclined relative to the horizontal line, both ends ofthe kiln are rotatably supported by the upper and lower side supportingmembers respectively, and the rotary kiln is rotatably driven through adriving apparatus such as a motor and a gear attached to the outputshaft of the driving apparatus with a ring-shaped gear mounted on theperiphery of the kiln.

In case of using a rotary kiln, a preferred embodiment is as follows.The temperature in the rotary kiln duct is defined as 600 to 1,000° C.and the rotary kiln is previously heated to the temperature. Then ceriumcarbonate hydrate is charged into the kiln duct at a predeterminedweight per hour, and the rotary kiln is rapidly heated at a temperaturerise rate of 20 to 200° C./min. At that time, the filling rate of ceriumcarbonate relative to the cross-sectional area of the kiln duct isdefined as 3 to 10%. Further, a predetermined flow rate of oxygen gas orthe like is blown into the kiln duct and the heating of the hydrate iscarried out in an oxidizing atmosphere.

It is preferred that an abrasive according to the present inventioncontains a slurry in which cerium oxide particles having an intensityratio of the area of a primary peak appearing at 27 to 30° to that of asecondary peak appearing at 32 to 35° (primary peak/secondary peak) of3.20 or more in a powder X-ray diffraction chart are dispersed into amedium.

In cerium oxide particles dispersed in the slurry in a cerium oxideabrasive of the present invention, the intensity ratio of the area of aprimary peak appearing at 27 to 30° to that of a secondary peakappearing at 32 to 35° (primary peak/secondary peak) is preferably 3.20or more, more preferably 3.20 to 4.20, and most preferably 3.30 to 4.00from the powder X-ray diffraction chart. Here, the diffraction angle orpeak intensity of scattered X-ray obtained by the powder X-raydiffraction reflects the properties relating to atoms which constitutethe crystal and their arrangement, and identification of a crystallinesubstance and structural analysis of crystallizability or the like canbe made from the diffraction chart.

The cerium oxide according to the present invention exhibits a cubicsystem and the primary peak appearing at 27 to 30° in the powder X-raydiffraction chart is analyzed as a [1,1,1] plane and the secondary peakappearing at 32 to 35° therein is analyzed as a [2,0,0] plane.

When a strain is caused by oxygen defect or the like in the crystal ofthe cerium oxide particles, the strain toward the [1,1,1] plane isincreased and the main peak intensity for 27 to 30° is decreased. Thus,the intensity ratio of the area of a primary peak appearing at 27 to 30°to that of a secondary peak appearing at 32 to 35° (primarypeak/secondary peak) is decreased. If the intensity ratio of the area ofa primary peak appearing at 27 to 30° to that of a secondary peakappearing at 32 to 35° (primary peak/secondary peak) is less than 3.20,the polishing speed can be rapidly decreased in some cases according tofilm properties of a surface to be polished.

Here, as a measuring device for the powder X-ray diffraction chart, acommercially available device (for example, Geigerflex, trade name,produced by Rigaku) can be used.

The larger the primary grain diameter of the cerium oxide particle is,and the less the crystalline strain is, i.e., the better thecrystallizability is, the higher speed polishing becomes possible withrespect to an SiO₂ insulating film formed by TEOS-CVD method or thelike. However, polishing scratches are likely to occur. Accordingly, thecerium oxide particles in the present invention are preferably preparedwithout enhancing the crystallizability thereof significantly. Further,since the cerium oxide abrasive is used for polishing semiconductorchips, the content of alkali metals and halogens in cerium oxide ispreferably limited to 1 ppm or less.

In the abrasive of the present invention, the content of Na, K, Si, Mg,Ca, Zr, Ti, Ni, Cr, and Fe is preferably each 1 ppm or lessrespectively, and the content of Al is preferably 10 ppm or less.

The cerium oxide particles according to the present invention can beproduced by calcining, for example, a cerium compound. However,calcining at a low temperature that does not increase thecrystallizability of the cerium particle as much as possible ispreferably used for preparing cerium oxide particles, which do not causescratches on the surface thereof.

The cerium oxide obtained by calcining can be ground by dry grindingwith a jet mill or the like, or by wet grinding with a bead mill or thelike. The ground cerium oxide particles include single crystallineparticles having a small crystalline size, and ground particles whichhave not been yet ground to the crystalline size. The ground particlesare different from an aggregate obtained by reaggregating the singlecrystalline particles, and comprise two or more crystallites havinggrain boundaries. When polishing is performed by an abrasive containingthe ground particles having the grain boundaries, the stress onpolishing breaks the boundaries and it is assumed that an active surfaceof crystal is continuously generated. Thus, a surface to be polishedsuch as an SiO₂ insulating film can be polished at high speed withoutcausing scratches.

The cerium oxide abrasive according to the present invention containscerium oxide produced by the above-mentioned method for producing ceriumoxide and pure water.

The cerium oxide abrasive according to the present invention can beobtained by mixing the cerium oxide particles produced by theabove-mentioned method, pure water and a dispersant used as requiredthereby to disperse the cerium particles. The cerium oxide particles maybe, if necessary, classified with a filter or the like. Here, while theconcentration of cerium oxide particles is not restricted, it ispreferably in a range of 0.1 to 10% by weight, and more preferably in arange of 0.5 to 10% by weight from the viewpoint of easy handling of asuspension (an abrasive).

Dispersants, which can disperse cerium oxide particles into a medium,may be used without limitation. The dispersants, which do not containmetallic ions, may include, for example, (meth)acrylic acid polymer andits ammonium salts; water-soluble organic polymers such as polyvinylalcohol, etc.; water-soluble anionic surfactants such as ammonium laurylsulfate, polyoxyethylene lauryl ether ammonium sulfate, etc.;water-soluble nonionic surfactants such as polyoxyethylene lauryl ether,polyethylene glycol monostearate, etc.; and water-soluble amines such asmonoethanolamine, diethanolamine, etc. (Meth)acrylic acid in the presentinvention means an acrylic acid and a methacrylic acid correspondingthereto, and alkyl (meth)acrylate means an alkyl acrylate and an alkylmethacrylate corresponding thereto.

Further, the acrylic acid polymers and their ammonium salts may include,for example, an acrylic acid polymer and its ammonium salt, amethacrylic acid polymer and its ammonium salt, and a copolymer ofammonium (meth)acrylate and alkyl (methyl, ethyl or propyl)(meth)acrylate.

Specifically, poly(ammonium (meth)acrylate) and a copolymer of ammonium(meth)acrylate and methyl (meth)acrylate are preferred. In case thelatter is used, the molar ratio of the ammonium (meth)acrylate to themethyl (meth)acrylate, that is, ammonium (meth)acrylate/methyl(meth)acrylate is preferably 10/90 to 90/10.

Further, the acrylic acid polymer or its ammonium salt preferably has aweight average molecular weight (value obtained by measuring with a GPCand calculated in terms of standard polystyrene) of 1,000 to 20,000 andmore preferably 5,000 to 20,000. When the weight average molecularweight exceeds 20,000, the change in grain size distribution with thelapse of time due to reaggregation tends to occur. On the other hand,when the weight average molecular weight is less than 1,000, the effectsof dispersibility and anti-sedimentation are sometimes insufficient.

The amount of these dispersants to be added preferably ranges from 0.01part by weight to 5 parts by weight based on 100 parts by weight ofcerium oxide particles from the viewpoint of dispersibility andanti-sedimentation properties of particles in slurry. To enhance thedispersing effect, it is preferred to charge the dispersantssimultaneously or substantially simultaneously into a dispersion machinetogether with the cerium oxide particles during the dispersion process.In a case where less than 0.01 part by weight of the dispersant is usedbased on 100 parts by weight of cerium oxide particles, the cerium oxideparticles tend to sediment, and on the other hand, in a case where morethan 5 parts by weight of the dispersant is used, the change in grainsize distribution with the lapse of time due to reaggregation tends tooccur.

In a method of dispersing these cerium oxide particles into water, ahomogenizer, an ultrasonic dispersing machine, a ball mill or the likemay be used in addition to a usual stirrer.

To disperse cerium oxide particles of a sub-μm order, it is preferred touse a wet type dispersion machine such as a ball mill, an oscillatingball mill, a planetary ball mill, and a medium stirring mill.

If the alkalinity of slurry is to be enhanced, an alkaline substancecontaining no metallic ion such as aqueous ammonia may be added duringthe dispersion process or after the process.

To the abrasive according to the present invention,N,N-diethylethanolamine, N,N-dimethylethanolamine,aminoethylethanolamine, anionic surfactants or the above-mentioneddispersants or the like may be added appropriately according to themanner of usage.

The aspect ratio of a primary particle, i.e., a crystallite, which formsthe cerium oxide particles dispersed in the cerium oxide abrasiveaccording to the present invention, is preferably 1 to 2, and a medianvalue of 1.3. The aspect ratio can be measured by observation with ascanning type electron microscope (for example, Model S-900 manufacturedby Hitachi, Ltd.).

In this abrasive, the cerium oxide particle preferably comprises 2 ormore crystallites, and has grain boundaries.

The median value of diameters of cerium oxide particles having grainboundaries is preferably 60 to 1,500 nm, more preferably 100 to 1,200rim, and most preferably 300to 1,000 nm.

The median value of the diameters of the crystallites is preferably 5 to250 nm, and more preferably 5 to 150 nm.

Particles having the median value of particle diameters of cerium oxidewith grain boundaries of 300to 1,000 nm, and the median value ofcrystalline diameters of 10 to 50 nm are preferably used.

Further, particles having the median value of particle diameters ofcerium oxide with grain boundaries of 300to 1,000 nm, and the medianvalue of diameters of crystallites of 50 to 200 nm are preferably used.

The maximum diameter of a cerium oxide having grain boundaries ispreferably 3,000 nm or less and the maximum diameter of crystallites ispreferably 600 nm or less, and more preferably 10 to 600 nm.

In the present invention, the crystalline particle diameter and ceriumoxide diameter having crystal particle boundaries can be measured byobservation with the above described scanning type electron microscope(for example, Model S-900 manufactured by Hitachi, Ltd.). The diameterof the cerium oxide particle, or a slurry particle, can be measured by alaser diffractometry (using, for example, Master Sizer microplusproduced by Malvern Instrument Co. Ltd.; refractive index: 1.9285, lightsource: He—Ne laser, absorption 0). Furthermore, the particle diameterof a particle can be obtained from the major axis and the minor axis ofthe particle. That is, the major axis and the minor axis of the particleare measured and the root of the product of major axis and the minoraxis of the particle is defined as a particle diameter. Then, the volumeof a sphere obtained by the resultant particle diameter is defined as aparticle volume.

The median value is one in the distribution of the volume particlediameter, and means a particle diameter when the volume ratio becomes50% after totalizing the volumes of particles from the smaller diameterof particle.

In a case where the cerium oxide particle is constituted by 2 or morecrystallites and has crystal particle boundaries, it is preferred thatcerium oxide particles, each of which has a particle diameter of 1 μm ormore, occupy 0.1% by weight or more of the total weight of the ceriumoxide particles. Such cerium oxide particles can polish a predeterminedsubstrate while being broken during polishing.

The measurement of the content of cerium oxide particles having aparticle diameter of 1 μm or more is performed by measuring theintensity of transmitted light shielded by particles using an in-liquidparticle counter. As a measuring device, a commercially available device(for example, model 770 Accu-Sizer (trade name) produced by, ParticleSizing System Inc.) can be used.

The cerium oxide particle constituted by 2 or more crystallites havinggrain boundaries preferably polishes a predetermined substrate whilepresenting,a new surface which is not yet in contact with a mediumduring polishing.

Further, it is preferable that the cerium oxide particle constituted by2 or more crystallites having grain boundaries has a ratio of thecontent of cerium oxide particles having a particle diameter of 0.5 μmor more measured by a centrifugal sedimentation process after thepolishing of a predetermined substrate to the content of cerium oxideparticles having a particle diameter of 0.5 μm or more measured by acentrifugal sedimentation process before the polishing of 0.8 or less.

Further, it is preferred that the cerium oxide particle constituted by 2or more crystallites having grain boundaries has a ratio of the ceriumoxide particle diameter of D99% by volume measured by a laserdiffractometry after the polishing of a predetermined substrate to thecerium oxide particle diameter of D99% by volume measured by a laserdiffractometry before the polishing of 0.4 or more and 0.9 or less.

Further, it is preferable that the cerium oxide particle constituted by2 or more crystalline grains having grain boundaries has a ratio of thecerium oxide particle diameter of D90% by volume measured by a laserdiffractometry after the polishing of a predetermined substrate to thecerium oxide particle diameter of D90% by volume measured by a laserdiffractometry before the polishing of 0.7 or more and 0.95 or less.

Here, the D99% and D90% mean particle diameters when the particlediameters become 99% and 90% respectively, after totalizing the volumesof particles from the smaller diameter of particle, in the distributionof the volume particle diameter.

Incidentally, the centrifugal sedimentation method is to measure theintensity of light transmitted through cerium oxide particles settled bycentrifugal force to obtain the content of the cerium oxide particles.As the measuring apparatus, for example, SA-CP4L (trade name) producedby Shimadzu Corp.) may be used.

Further, a state after a predetermined substrate has been polished meansthe state after a predetermined substrate is set on a holder on which asubstrate-mounting adsorption pad for supporting the substrate to bepolished, and the holder is placed on a platen to which a piece ofporous urethane resin polishing cloth is stuck with the surface to bepolished down, further a weight is placed thereon so that the workingload reaches 300 g/cm² and the surface to be polished is polished byrotating the platen for a predetermined period of time at a rotationspeed of 30 min⁻¹ (30 rpm) while dropping the abrasive on the platen ata dropping rate of 50 ml/min. At that time, the abrasive used forpolishing is circulated for reuse. The total amount of the abrasive is750 ml.

The measurement by a laser diffractometry can be performed with MasterSizer microplus (trade name) manufactured by Malvern Instrument Co.,Ltd. (refractive index: 1.9285, light source: He—Ne laser).

The abrasive according to the present invention contains a slurry inwhich abrasive grains having pores are dispersed into a medium. Here, asthe abrasive grains cerium oxide particles are preferably used.

The pore preferably has a pore ratio of 10 to 30% obtained from theratio between the density measured using a pycnometer and a theoreticaldensity obtained by the X-ray Rietveld analysis. A pore volume measuredby B. J. H. (Barret, Joyner, Halende) method is preferably 0.02 to 0.05cm³/g.

Further, the abrasive according to the present invention contains aslurry in which cerium oxide particles having a bulk density of 6.5g/cm³ or less are dispersed into a medium. Here, the density ispreferably 5.0 g/cm³ or more and 5.9 g/cm³ or less, and as the medium,pure water is preferably used. This slurry can contain a dispersant, andas a dispersant, at least one selected from a water-soluble inorganicpolymer, a water-soluble anionic surfactant, a water-soluble nonionicsurfactant and a water-soluble amine is preferred, and a salt of apolyacrylic acid polymer can be preferably used, and an ammonium saltthereof can be more preferably used.

The pH of the abrasive according to the present invention is preferably7 to 10, and more preferably 8 to 9 in the points of polishingproperties, dispersibility of cerium oxide particles, anti-sedimentationproperties and the like.

Films to be polished by the abrasives according to the present inventionmay include, for example, inorganic insulating films, specifically, anSiO₂ film formed by the CVD method using SiH₄ or tetraethoxysilane(TEOS) as an Si source and oxygen or ozone as an oxygen source.

As a predetermined substrate, a semiconductor substrate in a phase ofcircuit elements and aluminum wirings formed thereon or a semiconductorsubstrate in a phase of circuit elements formed thereon and further anSiO₂ insulating film layer formed thereon can be used. Also, a substrateincluding an SiO₂ insulating film formed for the purpose ofsemiconductor isolation (Shallow trench isolation) can be used.

By polishing an SiO₂ insulating film layer formed on such asemiconductor substrate with the above-mentioned abrasive, depressionsand projections on the surface of the SiO₂ insulating film layer areremoved thereby to form a smooth surface over the entire surface of thesemiconductor substrate.

Here, as a polishing device, a typical polishing device having a platen(on which a motor or the like whose number of revolutions is changeableis mounted) to which a holder, which holds a semiconductor device, and apiece of polishing cloth (a pad) are adhered, can be used.

As the polishing cloth, a general nonwoven fabric, an expandedpolyurethane, porous fluorine resins or the like can be used withoutspecific limitation. Further, it is preferred that in the polishingcloth a groove in which slurry is stored be formed.

Although the polishing conditions have no limitation, a low rotationalspeed of 100 min⁻¹ is preferred for the rotational speed of a platen sothat a semiconductor does not come off, and the pressure applied to thesemiconductor substrate is preferably 10⁵ Pa (1 kg/cm²) or less so thatno scratches will be present after polishing.

During polishing, a slurry is continuously supplied to the polishingcloth with a pump or the like. Although the supply amount of the slurryis not limited, it is preferred that the surface of the polishing clothbe always covered with the slurry.

Preferably, the polished semiconductor substrate should be rinsed wellin running water and water drops attached onto the semiconductorsubstrate should be shaken off and dried by using a spin dryer or thelike. On an SiO₂ insulating film layer smoothed in this manner, analuminum wiring, which is the second layer, is formed, then an SiO₂insulating layer is formed again between the wires and on the wiring bythe above-mentioned method, and subsequently the recessions andprojections on the surface of the insulating layer are removed bypolishing them with the above-mentioned cerium oxide abrasive, whereby asmooth surface is formed over the entire surface of the semiconductorsubstrate. By repeating this step with predetermined times, asemiconductor having a desired number of layers can be manufactured.

Predetermined substrates according to the present invention include asubstrate on which an SiO₂ insulating film is formed, a wiring board onwhich an SiO₂ insulating film is formed, an inorganic insulating filmsuch as glass and silicon nitride, an optical glass such as a photomask,a lens and a prism, an inorganic conducting film such as ITO, an opticalintegrated circuit, an optical switching element, an optical waveguideformed by glass and crystalline materials, an end surface of opticalfiber, optical single crystal for a scintillator or the like, a solidlaser single crystal, an LED sapphire substrate for blue laser, asemiconductor single crystal such as SiC, GaP, GaAs, etc., a glasssubstrate for a magnetic disc, a magnetic head and the like. The ceriumoxide abrasive according to the present invention is used for polishingthe above-mentioned substrates.

EXAMPLES

Next, the present invention will be described in detail by Examples.

Example 1 Preparation of Cerium Oxide Abrasive

A cerium carbonate hydrate was charged into a rotary kiln (kilndiameter: φ250 mm, kiln length L: 4000 mm) at a charging rate of 9 kgevery hour, and was calcined for one hour while blowing air at atemperature of 800° C. (the temperature rise speed at which the chargedcerium carbonate hydrate rises to a calcining temperature of 800° C. was53° C./min). 1 kg of the cerium oxide powder thus obtained wasdry-ground using a jet mill to obtain cerium oxide particles. 1 kg ofthe cerium oxide particles, 23 g of an aqueous ammonium polyacrylatesolution (40% by weight), and 8,977 g of pure water were mixed together,and ultrasonic dispersion was performed for 10 minutes while stirringthe mixture. The obtained slurry was filtered with a 1 μm filter andpure water was further added to the slurry to obtain 3% by weight of anabrasive.

A primary particle diameter of the cerium oxide particles was observedand measured with the above-mentioned scanning type electron microscope(S-900 manufactured by Hitachi, Ltd.). The particles had diameters of 20to 500 nm with the volume distribution median value of 80 nm. Further,the aspect ratio of the primary particle was a median value of 1.3.Moreover, a secondary particle diameter was measured using a laserdiffractometry (measuring device: Master Sizer microplus manufactured byMalvern Instrument Co. Ltd., measured with refractive index: 1.9285,light source: He—Ne laser, adsorption: 0) and the median value was 300nm and the maximum particle diameter was 2,600 nm. Further, the bulkdensity of these particles was measured using a picnometer, resulting in5.25 g/cm³. Further, the ideal density by an X-ray leat belt analysiswas 7.200 g/cm³. A pore ratio was calculated from these values, and itwas 27.1%. Additionally, the fine pore volume was measured by B.J.H.method. As a result, it was 0.040 cm³/g.

In the same manner, a measurement was performed with particles taken outof the abrasive. As a result, the intensity ratio of the area of aprimary peak appearing at 27 to 30° to that of a secondary peakappearing at 32 to 35° (primary peak/secondary peak) was 3.35.(Polishing of insulating film layer)

An SiO₂ insulating film (thickness: 1.5 μm) formed on a 8″ wafer byTEOS-plasma CVD method was polished for one minute with a CMP polishingdevice (Type EPO-111, manufactured by Ebara Co.). Polishing wasperformed while dropping a cerium oxide slurry (solid content: 1% byweight) at a dropping rate of 200 ml/min under the polishing conditionsof load: 30 kPa, back pressure: 15 kPa, revolution number of platen: 75min⁻¹, revolution number of carrier: 75 min⁻¹.

After polishing, the wafer was removed from the holder and cleaning ofthe wafer was performed by pure water brush washing using a PVA spongebrush for one minute, megasonic pulse cleaning for one minute, purewater rinsing for one minute, and spin drying for one minute with acleaning device (wafer scrubber manufactured by Tokyo Microtec Co.).Wafer thickness before and after polishing was measured at 49 points inthe wafer with a film thickness measuring device (Optical InterferometerLAMBDA Ace VLM8000-LS manufactured by Dai Nippon Screen Production Co.)and the polishing speed was obtained by dividing the difference betweenthe average value of the film thickness before polishing and that of thefilm thickness after polishing by the polishing time. As a result, thepolishing speed by this polishing was 5325 Å/min, and it was found thatthe entire wafer surface had a uniform thickness. Foreign substances anddefects on the polished wafer were detected with a particle counter(SFS6220 surface foreign substance detector manufactured by KLA Ten CallCo.) and the foreign substances and defects were observed by a reviewstation (AL2000 wafer appearance inspection microscope manufactured byOlympus Optical Co., Ltd., magnification: 800 to 1600 times, dark fieldobservation) based on all of the coordinate data on the wafer. Then, theforeign substances and defects were classified and the number ofscratches caused by polishing was obtained. As a result, the polishingscratches on one wafer were less than 30. Further, the particlediameters of the abrasive A after polishing were measured with acentrifugal sedimentation type grain size distribution meter. As aresult, the ratio of the content (% by volume) of a polished particlehaving a diameter of 0.5 μm or more to that of before polishing was0.455. Further, the particle diameters of the abrasive after polishingwere measured with a laser scatter type grain size distribution meter,and the obtained diameters of D99% and D90% were 0.521 and 0.825respectively, based on the values before polishing.

Example 2 (1) Preparation of Cerium Oxide Particles

a. Preparation of Cerium Oxide Particles A

2 kg of cerium carbonate hydrate was charged into a platinum vessel, andthe vessel was heated at a temperature rise rate of 80° C./min. Then,the hydrate was calcined at 800° C. for 2 hours while blowing air (10L/min) thereby to obtain about 1 kg of yellowish white powder. Thispowder was phase-identified by an X-ray diffractometry whereby it wasconfirmed to be cerium oxide.

The particle diameter of the resultant calcined powder was 30 to 100 μm.When a surface of the calcined powder particle was observed by ascanning type electron microscope, grain boundaries were found. When thecrystallite diameter of cerium oxide each surrounded by a grain boundarywas measured, the median value of the distribution was 190 nm and themaximum value was 500 nm.

1 kg of this cerium oxide powder was dry-ground using a jet mill. Theobservation of this ground particles was made with a scanning typeelectron microscope and not only small size particles having the samesize as that of the crystallite diameter but also large sizepolycrystalline particles of 1 to 3 μm and polycrystalline particles of0.5 to 1 μm were mixed with each other. The polycrystalline particleswere not agglomerate of single crystalline particles. The cerium oxideparticles obtained by grinding are hereinafter referred to as ceriumoxide particles A.

Then, when the primary particle diameter of the cerium oxide particles Awas observed and measured by a scanning type electron microscope, it was50 to 500 nm and the volume distribution median value was 90 nm.Further, the aspect ratio of the primary particle was the median valueof 1.2. Additionally, when the secondary particle diameter was examinedusing a laser diffractometry, the median value was 350 nm and themaximum particle diameter was 2800 nm.

b. Preparation of Cerium Oxide Particles B

2 kg of cerium carbonate hydrate was charged into a platinum vessel, andthe vessel was heated at a temperature rise rate of 70° C./min. Then,the hydrate was calcined at 700° C. for 2 hours while blowing air (10L/min) thereby to obtain about 1 kg of yellowish white powder. Thispowder was phase-identified by an X-ray diffractometry whereby it wasconfirmed to be cerium oxide.

The particle diameter of the resultant calcined powder was 30 to 100 μm.When a surface of the calcined powder particle was observed by ascanning type electron microscope, grain boundaries of cerium oxide werefound. When the crystallite diameter of cerium oxide each surrounded bya grain boundary was measured, the median values of the distribution was50 nm and the maximum value was 100 nm.

1 kg of this cerium oxide powder was dry-ground using a jet mill. Theobservation of this ground particles was made with a scanning typeelectron microscope and not only small size particles having the samesize as that of the crystal diameters but also large sizepolycrystalline particles of 1 to 3 μm and polycrystalline particles of0.5 to 1 μm were mixed with each other. The polycrystalline particleswere not agglomerate of single crystalline particles. The cerium oxideparticles obtained by grinding are hereinafter referred to as ceriumoxide particles B.

Further, the primary particle diameter of the cerium oxide particles Bwas 60 to 550 nm, and the volume distribution median value of 80 nm.Further, the aspect ratio of the primary particle was the median valueof 1.15. Additionally, when the secondary particle diameter was examinedusing a laser diffractometry, the median value was 400 nm and themaximum particle diameter was 2900 nm.

(2) Preparation of Abrasive

1 kg of the cerium oxide particles A or B obtained in theabove-mentioned step (1), 23 g of an aqueous ammonium polyacrylatesolution (40% by weight) and 8,977 g of deionized water were mixed andultrasonic dispersion was performed for 10 minutes while stirringthereby to disperse the cerium oxide particles. As a result, a slurrywas obtained.

The obtained slurry was filtered with a 1 micron-filter, and deionizedwater was further added to the slurry to obtain 3% by weight of anabrasive. The pHs of both slurries using the cerium oxide particles Aand B were 8.3. The abrasive obtained from the cerium oxide particles Ais hereinafter referred to as an abrasive A. On the other hand, theabrasive obtained from the cerium oxide particles B is hereinafterreferred to as an abrasive B.

The abrasive A or B was diluted at a suitable concentration and dried totake out particles in the slurry. Powder X-ray diffraction measurementof particles taken out from the abrasive A was made and the intensityratio of the area of a primary peak appearing at 27 to 30° to that of asecondary peak appearing at 32 to 35° (primary peak/secondary peak) wascalculated from the diffraction chart, resulting in 3.36. Further, theparticles were observed with the scanning electron microscope and thepolycrystalline particle diameters were measured. The median value ofthe diameter of the particles was 825 nm and the maximum value thereofwas 1230 nm. Then, the bulk density of this particles was measured witha pycnometer and the measured value was 5.78 g/cm³. Additionally, theideal density by an X-ray Rietveld analysis was 7.201 g/cm³. A poreratio was calculated from these values, resulting in 19.8%. Further, thefine pore volume was measured by B. J. H. method, and the resultantvalue was 0.033 cm³/g.

Also, the particles removed from the abrasive B were measured, and theintensity ratio of the area of a primary peak appearing at 27 to 30° tothat of a secondary peak appearing at 32 to 35° (primary peak/secondarypeak) exhibited 3.52.

(3) Polishing of Insulating Films

An Si wafer on which an SiO₂ insulating film (film I) was formed byTEOS-plasma CVD method was adsorbed on a substrate mounting adsorptionpad adhered to a holder to fix thereto. This holder was placed on aplaten to which a porous polyurethane resin polishing pad is adheredwith an insulating film surface down while holding the Si wafer, and aweight was placed thereon so that the working load reaches 300 g/cm².

Next, while dropping an abrasive A (solid content: 3% by weight)prepared in this Example on a platen at a dropping speed of 50 ml/min,the platen was rotated at the rotary speed of 30 min⁻¹ (30 rpm) for 2minutes to polish the insulating film. After polishing, the wafer wastaken away-from the holder and washed sufficiently with running water,and the wafer was cleaned for further 20 minutes with an ultrasoniccleaner. After cleaning the wafer, drops of water were removed with aspin dryer and the wafer was dried at 120° C. with an oven.

With regard to the dried wafer, film thickness change before and afterpolishing was measured with an optical a interference type filmthickness measuring device. As a result, it was found that in a casewhere the abrasive A was used, a 450 nm thick (polishing speed: 225nm/min) insulating film was polished and the wafer had uniform thicknessover the entire surface thereof. Further, when the surface of theinsulating film was observed with an optical microscope, no clearscratch was found.

With a wafer on which an SiO₂ insulating film (film II) made byTEOS-plasma CVD method was formed by using a different device from theabove-mentioned one, and with a wafer on which an SiO₂ insulating film(film III) prepared by a thermal oxidation process was formed, polishingwas studied. As a result, in the case where the abrasive A was used,insulating films of 420 nm (polishing speed: 210 nm/min) and of 520 nm(polishing speed: 260 nm/min) were polished respectively, and it wasfound that in both cases, the entire wafers had uniform thicknesses.Further, when the surfaces of the insulating films were observed withthe optical microscope, no clear scratch was found in both cases.

In the same manner as mentioned above, polishing of an insulating film(film I) was studied while dropping the abrasive B (solid content: 3% byweight) prepared in the present Example on a platen at a dropping speedof 50 ml/min. As a result, it was found that a 430 nm thick (polishingspeed: 215 nm/min) insulating film was polished and the wafer had auniform thickness over the entire surface thereof. Further, when thesurface of the insulating film was observed with an optical microscope,no clear scratch was found. With a wafer on which an SiO₂ insulatingfilm (film II) made by TEOS-plasma CVD method was formed by using afurther different device, and with a wafer on which an SiO₂ insulatingfilm (film III) made by a thermal oxidation process was formed,polishing was studied. As a result, in the case where the abrasive B wasused, insulating films of 400 nm (polishing speed: 200 nm/min) and of490 nm (polishing speed: 245 nm/min) were polished respectively, and itwas found that in both cases, the entire surface of wafers had uniformthickness. Further, when the surfaces of the insulating films wereobserved with an optical microscope, no clear scratch was found in bothcases.

Further, using the abrasive A, an SiO₂ insulating film on the surface ofan Si wafer was polished in the same manner as mentioned above, and theparticle diameters of abrasive A after polishing were measured with acentrifugal sedimentation type grain size distribution meter. As aresult, the ratio of the content (% by volume) of particles afterpolishing having a diameter of 0.5 μm or more to that of particlesbefore polishing was 0.385. Further, when the particle diameters of theabrasive A after polishing were measured with a laser scatter type grainsize distribution meter, the obtained diameters of D99% and D90% were0.491 and 0.804, respectively, based on the values before polishing.

Comparative Example 1 (1) Preparation of Cerium Oxide Particles C

2 kg of cerium carbonate hydrate was charged into a platinum vessel, andthe vessel was heated at a temperature rise rate of 15° C./min. Then,the hydrate was calcined at 800° C. for 2 hours under reduced pressure(10 mmHg) to obtain about 1 kg of yellowish white powder. This powderwas phase-identified by an X-ray diffractometry whereby it was confirmedto be cerium oxide.

The particle diameter of the resultant calcined powder was 30 to 100 μmand grain boundaries of cerium oxide were observed.

1 kg of this cerium oxide powder was dry-ground using a jet mill as inExample 2. As a result, not only small size particles having the samesize as that of the crystallite diameters but also large sizepolycrystalline particles of 1 to 3 μm and polycrystalline particles of0.5 to 1 μm were mixed with each other. The polycrystalline particleswere not agglomerate of single crystalline particles. The cerium oxideparticles obtained by grinding are hereinafter referred to as ceriumoxide particles C.

Further, the primary particle diameter of the cerium oxide particles Cwas observed and measured with a scanning type electron microscope. As aresult, the measured value was 150 to 700 nm, with the volumedistribution median value of 250 nm. Further, the aspect ratio of theprimary particle was a median value of 1.6. Additionally, when thesecondary particle diameter was examined using a laser diffractometry,the median value was 1,100 nm and the maximum particle diameter was3,500 nm.

(2) Preparation of Abrasive C

1 kg of cerium oxide particle C obtained in the above-mentioned step(1), 23 g of an aqueous ammonium polyacrylate solution (40% by weight)and 8,977 g of deionized water were mixed to obtain 3% by weight of anabrasive as in Example 2. The pH of the slurry was 8.3. The abrasiveobtained from the cerium oxide particles C is hereinafter referred to asan abrasive C.

By using the above-mentioned abrasive C, the intensity ratio of the areaof a primary peak appearing at 27 to 30° to that of a secondary peakappearing at 32 to 35° (primary peak/secondary peak) of particles takenout as in the same manner as Example 2 was calculated thereby to obtainthe value of 3.01.

(3) Polishing of Insulating Film

Using the above-mentioned abrasive C, the polishing of the insulatingfilm (film I) was studied as in Example 2. The results are shown inTable 1 at the end of the table including the results of the abrasives Aand B in Example 2.

The entire surface of a wafer polished using the abrasive C was uniformin thickness and no clear scratch was found. However, a 360 nm thick(polishing speed: 180 nm/min) insulating film was ground by thispolishing, which was about 20% lower in value than in the case of theabrasive A or B of the present invention even though it was calcined atthe same temperature.

With a wafer on which an SiO₂ insulating film (film II) prepared by aTEOS-plasma CVD method was formed by using a different device, and witha wafer on which an SiO₂ insulating film (film III) prepared by athermal oxidation process was formed, polishing was studied. As aresult, in the case where the abrasive C was used, insulating films of250 nm (polishing speed: 125 nm/min) and of 520 nm (polishing speed: 260nm/min) were polished respectively, and the polishing speeds were twicedifferent at maximum due to the film properties of a surface to bepolished.

TABLE 1 Powder X-ray diffraction Abra- Integrated in- Polishing speed(nm/min) sive tensity ratio Film I Film II Film III Example 2 A 3.36 225210 260 B 3.52 215 200 245 Compara- C 3.09 180 125 260 tive example 1

Industrial Applicability

The method for producing cerium oxide according to the present inventioncan easily produce cerium oxide which is an essential component of acerium oxide abrasive capable of polishing a surface to be polished suchas an SiO₂ insulating film at high speed without causing scratches, withgood yield.

The cerium oxide abrasive of the present invention can polish a surfaceto be polished such as an SiO₂ insulating film at high speed withoutcausing scratches.

The abrasive of the present invention can polish a surface to bepolished such as an SiO₂ insulating film at high speed without causingscratches irrespective of film properties.

The method for polishing a substrate according to the present inventioncan polish a surface to be polished such as insulating films at highspeed without causing scratches irrespective of film properties.

The method for manufacturing a semiconductor device according to thepresent invention can manufacture a semiconductor excellent inreliability with good yield and high productivity.

What is claimed is:
 1. A method for producing cerium oxide comprisingrapidly heating a cerium salt to a calcining temperature of said salt ata temperature rise rate of 20° to 200° C./min, and thereafter calciningthe cerium salt to produce cerium oxide.
 2. The method for producingcerium oxide according to claim 1, wherein calcination is performed by arotary kiln.
 3. The method for producing cerium oxide according to claim2 wherein the calcining temperature is from 600 to 1,000° C. and acalcining time is from 30 minutes to 2 hours.